Abstract
The known solvent dependence of 1J(Cc,Hf) and 2J(C1,Hf) couplings in acetaldehyde is studied from a theoretical viewpoint based on the density functional theory approach where the dielectric solvent effect is taken into account with the polarizable continuum model. The four terms of scalar couplings, Fermi contact, paramagnetic spin orbital, diamagnetic spin orbital and spin dipolar, are calculated but the solvent effect analysis is restricted to the first term since for both couplings it is by far the dominant contribution. Experimental trends of Δ1J(Cc,Hf) and Δ2J(C1,Hf) Vs ε (the solvent dielectric constant) are correctly reproduced although they are somewhat underestimated. Specific interactions between solute and solvent molecules are studied for dimethylsulfoxide, DMSO, solutions considering two different one-to-one molecular complexes between acetaldehyde and DMSO. They are determined by interactions of type C=O---H---C and S=O---H---C, and the effects of such interactions on 1J(Cc,Hf) and 2J(C1,Hf) couplings are analyzed. Even though only in a semiquantitative way, it is shown that the effect of such interactions on the solvent effects, of Δ1J(Cc,Hf) and Δ2J(C1,Hf), tend to improve the agreement between calculated and experimental values. These results seem to indicate that a continuum dielectric model has not enough flexibility for describing quantitatively solvent effects on spin-spin couplings. Apparently, even for relatively weak hydrogen bonding, the contribution from “direct” interactions is of the same order of magnitude as the “dielectric” effect.
Highlights
Two-bond coupling constants, 2J(X,Y), across a carbonyl carbon atom, Cc, are known [1] to be positive and their absolute values are uncommonly large
The modifications introduced in the Gaussian 98 program to calculate the PSO term are not adequate to calculate such a term taking into account the dielectric solvent effect
Results presented above suggest that spin-spin couplings strongly affected by negative hyperconjugations are adequate probes to study both inter- and intramolecular interactions that either inhibit or enhance such charge transfer interactions
Summary
Two-bond coupling constants, 2J(X,Y), across a carbonyl carbon atom, Cc, are known [1] to be positive (provided the magnetogyric ratios of both coupled nuclei are of the same sign) and their absolute values are uncommonly large Such features were rationalized in previous work [2] as originating mainly in the strong charge transfer interactions between the carbonyl oxygen lone pair of purely p character, n(p), and the Cc X and Cc Y antibonds, n(p)→(Cc X,Y)*. This suggests that this type of couplings should be sensitive to interactions that either enhance or inhibit such charge transfers; the former increasing the corresponding two-bond coupling, while the latter decreasing it. The known experimental values of the dependence of J(C1,Hf) and J(Cc,Hf) couplings in acetaldehyde on the dielectric solvent [4] makes it an adequate model compound to study what can be expected for larger molecular systems like proteins and nucleic acid bases
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